1. Field of the Invention
The present invention relates to a system for controlling an engine in a vehicle and an automatic transmission connected to the engine and, more particularly, to a control system for reducing a torsional torque, as might otherwise be left in a power transmission line.
2. Related Art
The power transmission line from the automatic transmission to the drive wheels of a vehicle is not completely rigid so that it elastically deforms. As a result, when the torque is inputted whereas a rotary member at the input side is fixed while the wheels being braked, the torsional deformation of the power transmission line is left as it is so that the torsional torque is accumulated. Since the automatic transmission of the vehicle is connected to the engine through a torque converter acting as a fluid coupling, as well known in the art, it is usual that the rotary member at the input side is not completely fixed. In a parking (P) range, however, the rotation of the output shaft of the automatic transmission is stopped by a parking gear and a parking lock pole meshing with the former, so that the torsional deformation of the power transmission line from the output shaft to the drive wheels is left.
When the torsional torque is accumulated in the P-range, it is released as the range is shifted from the P-range to another range to make such shocks or noises that the driver may feel uncomfortable. In the invention disclosed in JP-A-7-32915, therefore, the engine torque is reduced, when the vehicle is stopped in a reverse (R) range just before the shift to the P-range, thereby to reduce the torsional torque which might otherwise be accumulated in the power transmission line from the automatic transmission to the drive wheels.
In the system of the prior art described above, the engine torque is reduced under the two conditions that the R-range is set and that the vehicle is stopped. Of these, the R-range is generally detected in an electric manner. In the case of an electric failure, therefore, the control to reduce the engine torque cannot be executed because of the misjudgment of the range. Since the torsional torque to be left in the power transmission line increases in this case, it is abruptly released at the shift from the P-range to another range to make the shocks or noises.
Moreover, the aforementioned torsional torque resides not only in the shift to the P-range but also in the R-range in dependence upon the gear train. The torsional torque also resides in the automatic transmission which is constructed such that an auxiliary transmission unit constructed mainly of a set of planetary gear mechanism and adapted to be switched at two high and low stages is connected to the input side of a main transmission unit for setting a reverse stage and a plurality of forward stages, such that the auxiliary transmission unit is set to a directly connected stage or a low speed stage by a one-way clutch and to a high speed stage by applying a brake, and such that the auxiliary transmission unit is set to the high speed stage at the reverse stage. In the automatic transmission of this kind, more specifically, when an outputting rotary element in the auxiliary transmission unit is rotated backward while being set to the high speed stage by applying its brake, the one-way clutch is applied to integrate the auxiliary transmission unit as a whole so that the rotation is stopped in that state by the brake. In the R-range, therefore, the torsional torque acts in the direction to rotate the outputting rotary element of the auxiliary transmission unit backward so that the auxiliary transmission unit is left fixed as a whole to allow the torsional torque to reside.
When the engine torque is reduced while the vehicle is stopped in the R-range, the torsional deformation of the power transmission line can be suppressed to reduce the residual torsional torque. In this case, however, the engine torque cannot be reduced, when the range is misjudged because of the electric failure, and the residual torsional torque may increase.
Moreover, the construction is made to prefer the N-range when the two contacts (i.e., the contact for the R-range and the contact for the N-range) of a shift position sensor fail. Then, the control of the N-range is made even when the R-range is set in fact, so that the auxiliary transmission unit (or the overdrive unit) is directly connected in the R-range. As a result, the torque to be inputted to the main transmission unit may be increase to lower the durabilities of the clutches and/brakes in the main transmission unit.